IFF789@#1318: And last but not least, instrument displays are no longer anything like single signals going to single gauges, but rather a whole bunch of signals going into a chunk of software and coming out as a bunch of pixels. If some of those signals are so far outside normal flight that you are in a completely untested region, who knows what pixels that software will produce ? If there is a flaw, it could affect more than just VSI. Or put another way, that "needle" may not peg, it might wander off the VSI and corrupt some other part of the display.
What we have in the FDR traces are the input signals to the instruments. If it was steam gauges, I'd be happy to trust that the gauge was showing the same - invoking occam to say that we should look for simpler solutions than a failure of a gauge independent but simultaneous with other problems. The behaviour when the signal goes out of range (hits the stops) is also simple to test and be confident in. For glass / PDF I'm not so sure. Not at all.
Sadly, we'll never know exactly what they were looking at, but I would hope that at the very least the HF investigation is feeding the FDR signals into a sim (or the iron bird) to get as close as we can to what the instruments were showing. If we had video of that here, I have a suspicion it would be a bit more confusing than what we may be assuming from the FDR.